The present invention relates to a special multi-mode optical fiber (multimode fiber or MM fiber or fibre). A multi-mode optical fiber is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters—more than sufficient for the majority of premises applications.
The equipment used for communications over multi-mode optical fiber is much less expensive than that for single-mode optical fiber. Typical transmission speed/distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s to 220-550 m (1000BASE-SX), and 10 Gbit/s to 300 m (10 GBASE-SR).
Because of its high capacity and reliability, a multi-mode optical fiber generally is used for backbone applications in buildings. An increasing number of users are taking the benefits of fiber closer to the user by running fiber to the desktop or to the zone. Standards-compliant architectures such as Centralized Cabling and Fiber to the Telecom Enclosure offer users the ability to leverage the distance capabilities of fiber by centralizing electronics in telecommunications rooms, rather than having active electronics on each floor.
In order to minimize intermode dispersion, the multimode optical fibers used in telecommunications generally comprise a core of refractive index that decreases progressively going from the center of the fiber to its junction with the cladding.
An optical fibre having a refractive index profile is to be understood as meaning that the refractive index of the core of the optical fibre has a gradient index shape that complies with the formula:
      n    ⁡          (      r      )        =            n      1        ⁢                  (                  1          -                      2            ⁢                                          Δ                ⁡                                  (                                      r                    a                                    )                                            α                                      )            
being relevant for r<a in the core.
Herein                n(r)=refractive index at a radius r=r        n1=refractive index of the core at the fibre axis (r=0)        n2=refractive index of the (inner) cladding surrounding the core        Δ=normalised refractive index difference:        
  Δ  =                    n        1        2            -              n        2        2                    2      ⁢              n        1        2                            α=profile exponent, alpha value        a=core radius [μm]        r=distance from the axis of the fibre [μm]        The parameter Δ is known as the index contrast; and for Δ<<1,n(r≧a)=n0·(1−√{square root over (1−2Δ)})≈Δ·n0         
So-called “α profile” fibers and their method of fabrication are disclosed in document U.S. Pat. No. 3,989,350.
Multi-mode fibers are described by their core and cladding diameters. Thus, a 62.5/125 μm multi-mode fiber has a core size of 62.5 micrometers (μm) and a cladding diameter of 125 μm. In addition, multi-mode fibers are described using a system of classification determined by the ISO 11801 standard—OM1, OM2, and OM3—which is based on the bandwidth of the multi-mode fiber. OM4 (defined in TIA-492-AAAD) was finalized in August 2009, and was published by the end of 2009 by the TIA.
In fiber optics, a graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the fiber axis (the imaginary central axis running down the length of the fiber).
Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The advantage of the graded-index fiber compared to multimode step-index fiber is the considerable decrease in modal dispersion.
The most common refractive index profile for a graded-index fiber is very nearly parabolic (α≈2). The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.
This type of fiber is normalized by the International Telecommunications Union ITU-T at recommendation G.651: “Characteristics of a 50/125 μm multimode graded index optical fiber cable”
The present invention relates in general to temperature sensors, and in particular to a new class of optical fiber distributed temperature sensors suited to use in harsh, hydrogen-containing environments.
Various documents relating to optical fibres exist.
For instance, U.S. Pat. No. 4,741,747 (A), U.S. Pat. No. 7,519,256 (B2), US 2005120751 (A1), US 2005000253 (A1), US 2008063812 (A1), and US 2005041943 (A1) recite methods of manufacturing an optical fibre.
Also U.S. Pat. No. 6,853,798 B1 recites optical fibers.
One of the niche applications for fiber optics is distributed temperature sensing in geothermal wells. A temperature profile obtained shortly after drilling will determine potential of a well for use in power generation, and provides guidance as to how to best harness heat generated by the well.
Additionally, long-term thermal monitoring of a power-producing geothermal well is needed to operate the well so that production of electric power is optimized. Additional water must periodically be re-injected into the well, resulting in localized cooling. Efficient operation of a geothermal well often requires that the re-injection point be moved to a hotter region of the well. It is well known in the art that a vertical temperature profile of an entire geothermal well can be obtained essentially instantaneously using a single optical fiber. As a result, use of an optical-fiber distributed temperature sensing system as a geothermal well logging tool is held to offer much potential.
Operating principles of a typical optical fiber distributed temperature sensor follow below. When light of a frequency ω interacts with a medium in which molecular or lattice vibration is taking place at a frequency ωr said light will be Raman scattered from the medium. The scattered light will include frequencies of ω±ωr as well as the original frequency ω. A portion of this scattered light propagates opposite to the propagation direction of the incident light, or is back scattered.
Intensity of the various frequency components of the back scattered light are found to depend on the temperature of the medium at the point where the back scattered light is generated. Accordingly, proper detection and analysis of the back scattered light in a medium allows one to determine the distribution of temperature in that medium.
In a prior art optical fiber distributed temperature sensor, a light of a known frequency is introduced into an optical fiber whose temperature distribution along its length is to be measured. The back scattered light is collected, and spectral analysis of the back scattered light is carried out using time domain techniques, among others.
A result is a relationship between temperature of the back scattering medium and time. As the back scattering medium is part of the optical fiber, however, the time when the back scattered radiation is collected for analysis is directly related to the distance along the fiber where the back scattering medium is located. Thus, the relation between temperature and time can be easily converted into the desired relation between temperature and position along the fiber.
As mentioned before, use of an optical fiber distributed temperature sensor for monitoring and evaluation of geothermal wells is considered to be an attractive possibility in the art. For such use to be practical, however, requires that the optical fiber to be placed in the geothermal well can survive the harsh downhole environment for a period measured in years.
Field tests of optical fiber distributed temperature sensors in geothermal wells have demonstrated that conventional optical fibers are insufficiently robust for this type of application. In the hotter wells studies, anomalies associated with changes in the optical transmission characteristics of the optical fibers used were seen in as little as 24 hours. The optical fibers were rendered useless for the intended application within time periods far shorter than the required service life.
Transmission anomalies were found to relate to the formation of OH ions in the silicate glass matrix of the optical fibers. These OH ions did not exist in the optical fibers prior to their exposure to the downhole environment. The likely degradation mechanism is that hydrogen in the hot downhole environment diffuses into the fiber, and there reacts with the oxygen of the silicate glass to form OH ions.
The constituents of the glass are found to have a strong influence on the rate at which OH ions are formed in a typical downhole environment. Optical fibers typically have a core glass with a refractive index which is larger than that of a surrounding cladding glass. An optical fiber can have a step-index structure, where there is an essentially abrupt interface between the core and the cladding glasses, or can have a graded-index structure, where the properties of the fiber vary in a graded manner radially in the.
A common usage is to introduce germanium to increase the core refractive index, relative to the index of the surrounding cladding. It has been found, however, that the presence of germanium promotes the formation of OH ions in the downhole environment. As a consequence such is discouraged.
Phosphorous is also commonly added to the glass to improve manufacturing characteristics by reducing the viscosity of the glass. Phosphorous is found to promote the formation of OH ions to a greater extent than does germanium. Generally, then, commercially available optical fibers comprise materials which render them susceptible to hydrogen damage through OH ion formation.
The only solution to this problem which seems to have been explored by the geothermal industry is to introduce a hydrogen diffusion barrier at the surface of the optical fiber, to attempt to prevent diffusion of hydrogen into the fiber. Various barrier coatings, such as carbon, silicon oxynitride, and aluminum, have been investigated. Although such barrier coatings are found to be effective at low temperatures, their effectiveness largely disappears at higher temperatures, typically in excess of 250° C. As many geothermal applications involve exposure to environments hotter than this, such barrier coatings do not provide adequate protection for optical fiber distributed temperature sensors in geothermal applications.
Both phosphorous-free and germanium-free fibers have been tested in hot hydrogen-containing environments. However, even a step-index fiber with a pure silica core exhibits unacceptable levels of OH ion formation.
It is commonly held in the geothermal industry that routine usage of optical fiber-based sensors, and in particular distributed temperature sensors, as downhole instrumentation in geothermal wells is highly desirable. Other types of fiber-optic-based downhole sensors, such as interferometric strain and tilt sensors, are also desirable for use in hot, hydrogen-containing environments, but are difficult to implement owing to OH ion formation in the optical.
There therefore exists need for optical fiber-based sensors, and in particular for optical fiber distributed temperature sensors, which comprises an optical fiber sufficiently resistant to OH ion formation within the downhole environment that said fiber, and hence said sensor, has a service life of sufficient duration for the intended applications. Further the fiber and sensor should be less sensitive to hydrogen, and have a superior bandwidth.